MANILA, Philippines--The government-dominated National Assembly held the first meeting yesterday on its official vote canvass, which by law will determine who won the disputed presidential election here.

In the slow count of ballots cast last Friday, the government election commission showed President Ferdinand E. Marcos leading by 53 percent to 47 percent. An unofficial count by a citizens' poll-watching group of more votes showed challenger Corazon Aquino ahead by the same margin.

The 178-member National Assembly, made up largely of Marcos supporters, has 15 days to complete the canvass of vote tally sheets from 137 locales--74 provinces, 53 cities and 10 Manila district offices. A member of the ruling party in the assembly the job could be done in 48 hours if all precincts were on hand, but only 60 had been received yesterday.

Aquino Claims Victory

At a gathering in suburban Makati, Aquino had told 2,000 cheering supporters she was "claiming the people's due," and pledged: "We are going to take power. The people have won this election."

Aquino accuses Marcos of widespread election fraud in attempting to extend his 20 years of rule over this archipelago of 7,100 islands.

Both Aquino supporters and official U.S. election observers called the slow count an attempt by Marcos to manipulate the results. The American observers left for home yesterday.

In Washington, a senior Reagan Administration official appealed to Filipinos "not to have violence, not to have demonstrations in the street just because you didn't like the election [outcome]."

"Get on the team and work with the government to form a government, whether it's Marcos or Aquino," said the official, who spoke on condition of anonymity.

National Assembly members, two-thirds of whom are from the president's New Society Movement, spent four hours yesterday debating rules for the canvass and then adjourned until tomorrow afternoon. The galleries were packed with Marcos supporters and Aquino loyalists who chanted their candidates' names. Thousands more gathered outside.

Returns at the end of the day from the so-called quick count by the government commission gave Marcos 4,017,277 votes, or 53 percent, to 3,610,099, or 47 percent, for Aquino, with 35 percent of the precincts count.

A count by the National Movement for Free Elections, a poll-watchers' group known as Namfrel, had Aquino ahead by 6,658,838 votes to 5,971,693, a lead of 53 percent to 47 percent, with 60 percent of precincts reported.

The election commission's count was suspended after 30 computer operators walked out Sunday, charging fraud in the tabulation that showed Marcos leading.

Radio Veritas, a Roman Catholic Church station, said eight more of the commission's 120 computer technicians walked out yesterday.

The election was marred by violence, which continued yesterday. A gunman fired at about 50 Aquino supporters in an open truck from which Aquino had delivered a speech earlier, killing a 20-year-old man and wounding a woman.

Aquino had stood on the truck that later was fired upon to speak to supporters at an outdoor Catholic Mass.

A witness, who spoke on condition of anonymity, said the shots were fired from a car that pulled alongside at a traffic light and sounded as if the gun was equipped with a silencer. He said the car sped off after the shooting.

Witnesses said the dead man had been holding up a poster reading "Marcos Concede" when the bullet hit him in the chest, killing him instantly. The woman, 19, was wounded on the forehead.

More than 90 people have been killed in election-related violence since the campaign began Dec. 6.